UPDATE: AstraZeneca rejected Pfizer's revised proposal to acquire the company for 50 pounds per share ($84.47), saying it was "inadequate." Pfizer had earlier submitted a new proposal to buy AstraZeneca, with the offer including 1.845 shares in the combined company and 15.98 pounds ($27.00) in cash, valuing the UK drugmaker at 63 billion pounds ($106.5 billion).

Earlier this week, Pfizer confirmed that it was weighing its options regarding a possible merger with AstraZeneca having contacted the company twice about such a proposal, most recently last month. According to Pfizer, its first proposal made in January represented 46.61 pounds ($76.62) per AstraZeneca share and a premium of approximately 30 percent to the UK company's closing share price on January 3, valuing the transaction at around 58.8 billion pounds ($99 billion). AstraZeneca, which rejected the offer, saying it "very significantly undervalued" the company, noted that the initial proposal was made up of 30 percent cash and 70 percent of Pfizer shares.

Pfizer CEO Ian Read recently met with UK officials to assuage concerns about the potential transaction, with the executive stressing the company's desire to conduct R&D and manufacturing in the country as well as the appeal of the British tax system. UK lawmakers have said they will probe Pfizer's proposal to determine whether the offer would threaten jobs in the country.

Meanwhile, AstraZeneca chairman Leif Johansson has indicated that the drugmaker will return to growth as an independent company. Analysts have also suggested that AstraZeneca could pursue a merger of equals with companies such as Amgen or AbbVie as a defence strategy against a takeover by Pfizer.